Due to extremely poor electrical conductivity of all cathode active materials in a lithium-ion or lithium metal cell, a conductive additive (e.g. carbon black, fine graphite particles, expanded graphite particles, or their combinations), typically in the amount of 2%-15%, must be added into the electrode. However, the conductive additive is not an electrode active material. The use of a non-active material means that the relative proportion of an electrode active material, such as LiFePO4, is reduced or diluted. For instance, the incorporation of 5% by weight of PVDF as a binder and 5% of carbon black as a conductive additive in a cathode would mean that the maximum amount of the cathode active material (e.g., lithium cobalt oxide) is only 90%, effectively reducing the total lithium ion storage capacity. Since the specific capacities of the more commonly used cathode active materials are already very low (140-170 mAh/g), this problem is further aggravated if a significant amount of non-active materials is used to dilute the concentration of the active material.
State-of-the-art carbon black (CB) materials, as a conductive additive, have several drawbacks:                (1) CBs are typically available in the form of aggregates of multiple primary particles that are typically spherical in shape. Due to this geometric feature (largest dimension-to-smallest dimension ratio or aspect ratio˜1) and the notion that CBs are a minority phase dispersed as discrete particles in an electrically insulating matrix (e.g. lithium cobalt oxide and lithium iron phosphate), a large amount of CBs is required to reach a percolation threshold where the CB particles are combined to form a 3-D network of electron-conducting paths.        (2) CBs themselves have a relatively low electrical conductivity and, hence, the resulting electrode remains to be of relatively low conductivity even when the percolation threshold is reached. A relatively high proportion of CBs (far beyond the percolation threshold) must be incorporated in the cathode to make the resulting composite electrode reasonably conducting.        
Clearly, an urgent need exists for a more effective electrically conductive additive material. Preferably, this electrically conductive additive is also of high thermal conductivity. Such a thermally conductive additive would be capable of dissipating the heat generated from the electrochemical operation of the Li-ion battery, thereby increasing the reliability of the battery and decreasing the likelihood that the battery will suffer from thermal runaway and rupture. With a high electrical conductivity, there would be no need to add a high proportion of conductive additives.
There have been several attempts to use other carbon nano-materials than carbon black (CB) or acetylene black (AB) as a conductive additive for the cathode of a lithium battery. These include carbon nano-tubes (CNTs), vapor-grown carbon nano-fibers (VG-CNFs), and simple carbon coating on the surface of cathode active material particles. The result has not been satisfactory and hence, as of today, carbon black and artificial graphite particles are practically the only two types of cathode conductive additives widely used in lithium ion battery industry. The reasons are beyond just the obvious high costs of both CNTs and VG-CNFs. The difficulty in disentangling CNTs and VG-CNFs and uniformly dispersing them in a liquid or solid medium has been an impediment to the more widespread utilization of these expensive materials as a conductive additive. For the less expensive carbon coating, being considered for use in lithium iron phosphate, the conductivity of the carbon coating (typically obtained by converting a precursor such as sugar or resin via pyrolyzation) is relatively low. It would take a graphitization treatment to render the carbon coating more conductive, but this treatment requires a temperature higher than 2,000° C., which would degrade the underlying cathode active material (e.g., LiFePO4).
As an alternative approach, Ding, et al investigated the electrochemical behavior of LiFePO4/graphene composites [Y. Ding, et al. “Preparation of nano-structured LiFePO4/graphene composites by co-precipitation method,” Electrochemistry Communications 12 (2010) 10-13]. The co-precipitation method leads to the formation of LiFePO4 nano-particles coated on the surfaces of graphene nano-sheets. The cathode is then prepared by stacking these LiFePO4-coated graphene sheets together. This approach has several major drawbacks: (1) With the two primary surfaces of a graphene sheet heavily loaded with LiFePO4 nano-particles, the resulting electrode entails many insulator-to-insulator contacts between two adjoining coated sheets in a stack. (2) With both LiFePO4 particles and graphene sheets being nano-scaled (single-layer graphene is as thin as 0.34-1.0 nm), the coated sheets are also nano-scaled, making the preparation of electrodes very difficult. The LiFePO4 nano particles are notoriously difficult to be compacted into cathodes of desired dimensions using the current battery production equipment. We have found that the nano particle-coated nano graphene sheets as prepared by the co-precipitation method are not amenable to fabrication of cathodes with the same equipment as well. In particular, these coated nano graphene sheets could not be compacted into a dense state with a high mass per unit electrode volume. In other words, the cathode tap density is relatively low. This is a very serious issue since all of the commonly used cathode active materials, including LiFePO4, already have a very low specific capacity (mAh/g), and not being able to pack a large mass of a cathode active material into a given electrode volume would mean an excessively low overall capacity at the cathode side. (It may be noted that the typical specific capacity (140-170 mAh/g) of a cathode active material is already much lower than that (330-360 mAh/g) of an anode active material. Such an imbalance has been a major issue in the design and fabrication of lithium ion batteries.) This issue has been completely ignored by Ding, et al.
Thus, it is an object of the present invention to provide a thermally and electrically conductive additive that can be easily incorporated in a cathode of a lithium battery.
A specific object of the present invention is to provide a conductive additive that is capable of helping multiple primary particles of a cathode active material to aggregate into secondary particles (herein referred to as particulates) that are more amenable to mass production of cathodes using current production equipment. In other words, the conductive additive is also a modifier for other properties of an electrode.
A particularly desirable specific object of the present invention is to provide a conductive additive or modifier that is capable of helping multiple primary particles of a cathode active material aggregate into secondary particles that are more conducive to the formation of a 3-D network of electron-conducting paths, imparting exceptional conductivity to the cathode and enabling the cathode to become high-rate capable.